The Mediterranean Sea (Mare Nostrum)

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The empire's actual transport network. Bulk goods - grain, oil, wine, stone, amphorae - moved by ship because water transport was roughly an order of magnitude cheaper than road haulage per tonne-kilometre in the ancient world. Rome is the only empire in the region to have held every shore of this sea at once, and once it did, the sea stopped being a frontier and became an interior. The roads are best understood as feeders to and between ports, not as an alternative to them.